Electrodeposition of chromium-containing films on ferrous metal articles



United States Patent US. Cl. 204-56 15 Claims ABSTRACT OF THE DIStILOSURE The corrosion resistance of ferrous metal articles is improved by electrolytic treatment as a cathode in an acidic aqueous electrolyte containing a hexavalent chromium compound, a substance providing chloride ion, and a substance providing sulfate ion to deposit a corrosion resistant chromium-containing film thereon.

SPECIFICATION This invention relates to the electrochemical treatment of ferrous metal articles to impart improved corrosion resistance and other desirable properties thereto. The invention further relates to the corrosion resistant ferrous metal articles thus produced.

Tinplate is widely used for the manufacture of containers and it is generally satisfactory for this purpose. However, among other reasons, the high cost of tin and the lack of a dependable source of tin in this country have made it desirable to provide low cost tin-free steel strip or sheet for use as container stock.

A number of efforts have been made to provide satisfactory tin-free strip or sheet for use as container stock. Inasmuch as a clean steel surface is subject to rapid corrosion, some type of a protective coating must be applied at the time of manufacture, The initially protected steel surface of the strip or sheet must be receptive to the usual organic coatings such as lacquers and enamels which are applied to provide relatively permanent protection against corrosion, and it also should be receptive to the organic adhesives which are used in joining the side edge portions at the side seams of containers prepared therefrom. Thus, tin-free steel strip or sheet for use as container stock should have a surface which has a combination of desirable characteristics, including the followmg:

(A) The steel surface must have an initial protective film or coating thereon which retards corrosion at least from the time of manufacture up until the steel strip or sheet is used in the preparation of containers.

(B) The initially protected steel surface should have a pleasing bright silver to gray appearance, as distinguished from a dull and/or discolored appearance, as it is often desirable to apply clear organic protective coatings thereto.

(C) The initially protected steel surface must be receptive to organic protective coatings of the usual types applied to tinplate and must form a good base therefor. For example, the organic coatings must adhere tightly to the steel surface and provide good underfilrn corrosion resistance.

(D) The initially protected steel surface should pass the side seam adhesion test as the side seams of tin-free containers are often formed by cementing the side edges together with a resinous organic cement. If the overlapped edge portions cannot be tightly adhered together with the organic cement, the side seams separate and the containers fail.

The initially protected steel surface should meet all of the above requirements. Additionally, since this is a highly Patented Oct. 28, 1969 competitive field and the initially coated steel strip or sheet must compete wth other low cost materials, the process that is used in treating the steel surface must be inexpensive and not add appreciably to the overall cost of manufacture. A wide variety of electrochemical treatments and other methods of treatment have been proposed for protecting the steel surface initially. However, the treatments and/or coatings proposed heretofore have had some disadvantage or deficiency which prevented them from being entirely satisfactory.

It is an object of the present invention to provide a novel method of electrochemically treating ferrous metal articles to improve the corrosion resistance.

It is a further object to provide anovel method of electrochemically treating ferrous metal strip or sheet of a gauge satisfactory for container manufacture to provide a corrosion resistant surface thereon which is receptive to protective organic coatings, and which also has satisfactory side seam adhesion characteristics.

It is still a further object to provide a novel electrochemical treatment for the preparation of container stock from steel strip or sheet which does not discolor the treated steel surface.

It is still a further object to provide the improved corrosion resistant ferrous metal articles prepared by the method of the invention.

Still other objects and advantages of the invention will be apparent to those skilled in the art upon reference to the following detailed description and the examples.

In accordance With the present invention, ferrous metal articles are electrolytically treated as a cathode in an electrolyte consisting essentially of an acidic aqueous solution of a hexavalent chromium compound, a substance providing chloride ion, and a substance providing sulfate ion to deposit a corrosion resistant chromium-containing film thereon. There are certain preferred variants which produce superior results, as will be described in more detail hereinafter.

Any suitable water soluble hexavalent chromium compound which provides a species of chromate ion in solution may be used in practicing the invention. Examples of hexavalent chromium compounds include chromic acid (CF03) which is usually preferred, chromic acetate, alkali metal and ammonium chromates such as sodium chromate, sodium dichromate, potassium chromate, potassium dichromate, ammonium chromate and ammonium dichromate. In most instances, it is preferred that the electrolyte contain about 15-250 grams per liter, and preferably about 20-70 grams per liter of the hexavalent chromium compound. Best results are usually obtained when the electrolyte contains about 40-50 grams per liter of the hexavalent chromium compound.

It is essential that the electrolyte contain both chloride ion and sulfate ion. The water soluble substance providing chloride ion should be dissolved in the electrolyte in an amount to provide by weight 25-200 parts per million, and preferably about 50-150 parts per million of chloride ion. Examples of suitable water soluble chlorides include the alkali metal chlorides, the alkaline earth metal chlorides, ammonium chloride, and hydrogen chloride (hydrochloric acid). The preferred chlorides are usually hydrogen chloride- (hydrochloric acid), sodium chloride and potassium chloride, and for best results the selected chloride should be present in an amount to provide about -110 parts per million by weight of the electrolyte of chloride ion. The water soluble substance providing sulfate ion should be dissolved in the electrolyte in an amount to provide about -2000 parts per million by weight of sulfate ion, and preferably about 200-800 parts per million by weight. Suitable water soluble sulfates include the alkali metal sulfates, the alkaline earth metal sulfates, ammonium sulfate and hydrogen sulfate (sulfuric acid). Usually hydrogen sulfate (sulfuric acid), sodium sulfate or potassium sulfate is preferred, and for best results the selected sulfate should be present in an amount to provide 400-500 parts per million by weight of the electrolyte of sulfate ion. Complete ionization of the dissolved substances providing chloride ion and sulfate ion is assumed when calculating the weights of chloride ion and sulfate ion present in the electrolyte.

The ferrous metal article to be treated is immersed in the electrolyte and connected to a source of direct current as the cathode. An inert anode is also immersed in the electrolyte and electrically connected to the source of direct current. The article is then electrolytically treated as the cathode under current conditions whereby a colorless or substantially colorless film is deposited. The article may be treated as a cathode at the rate of, for example, 50-800 arnperes per square foot, and preferably about 100-600 arnperes per square foot, over a period of time sutficient to provide about 100-800 coulombs, and preferably 200-600 coulombs of current per square foot of surface area treated. The best results are often obtained when the current conditions are such that the article is cathodically treated at the rate of 200-400 arnperes per square foot over a period of time sufiicient to provide 300-400 coulombs of current per square foot.

The aqueous electrolyte should be strongly acidic and have a pH value less than 2.5, and preferably should have a pH value of 2 or less. For example, the pH of the electrolyte may be about 0.1-2, and preferably about 0.1-1. In instances where the electrolyte is prepared from chromic acid, hydrochloric acid, sulfuric acid and Water, the existing pH is very satisfactory. Thus, the preferred electrolyte is prepared from only these substances.

The temperature of the electrolyte should not exceed about 175 F., and may be, for example, about 70-175 F., and preferably about 90-150 F. The best results are usually obtained when the electrolyte has a temperature of about 100-120 F.

When a ferrous metal substrate is treated under the foregoing conditions, the resulting corrosion resistant film that is deposited thereon is composed of a mixture of metallic chromium and an oxide of chromium in which the chromium appears to have a valence of plus 3. The total amount of metallic chromium and chromium in the form of the oxide which is present in the film usually varies between about 2 and 25 milligrams per square foot, and is preferably within the range of about 5 and 15 milligrams per square foot. Best results are often obtained when the substrate has about 6-12 milligrams per square foot of total chromium present in the film. In most instances, metallic chromium makes up about 40- 95% of the total chromium present in the film, and preferably about 60-80%.

The present invention is especially useful in the treatment of blackplate strip or sheet of the usual gauges used in tinplate manufacture to thereby provide container stock for the preparation of containers for the preservation and storage of foods, beverages, and the like. However, the treatment may be applied to any ferrous metal surface that it is desired to impart corrosion resistance thereto.

The ferrous metal article to be treated need not be given a special pretreatment. However, the surface should be free of rolling oil, rust, dirt, scale and other foreign matter, and it is preferred that the article be cleaned by the prior art process used in the manufacture of tinplate. In instances where the substrate is blackplate strip, the strip may be electrolytically treated in a caustic cleaner under the prior art conditions, pickled in sulfuric acid or hydrochloric acid, rinsed, and then electrochemically treated under the conditions of the present invention. Thereafter, the strip may be rinsed with hot or cold water, dried, oiled lightly with dioctyl sebacate or other suitable lubricant used in tinplate manufacture, and coiled.

The corrosion resistant film that is deposited on the ferrous metal substrate does not discolor the surface. The treated surface is bright and varies between gray and silver-gray, and thus it is usually substantially the same color as the cleaned blackplate. This is of importance in container manufacture, as often it is desired to apply clear organic coatings such as enamels or lacquers thereto for the purpose of protecting the steel base against corrosion. The organic coatings may be the same as those applied to tinplate, such as phenolic lacquers, modified phenolic lacquers, epoxy based enamels, and the like.

The foregoing detailed description and the following specific examples are for purposes of illustration only, and are not intended as being limiting to the spirit or scope of the appended claims.

Example I Blackplate strip of tinplate gauge is cleaned by a prior art procedure to remove grease, dirt and other foreign matter from the surface. The treatment includes an electrolytic treatment in a standard caustic cleaner, rinsing, pickling in aqueous sulfuric acid, rinsing, and drying. The cleaned strip was cut into laboratory test panels and electrochemically treated in accordance with the invention.

The electrolyte contained 50 grams per liter of chromic acid (CrO parts per million by weight of chloride ion which was added as hydrochloric acid, and 500 parts per million of sulfate ion by weight which was added as sulfuric acid. The electrolyte contained no other ingredients, with the exception of the usual trace minerals in the water and the usual traces of impurities in the added ingredients.

The blackplate test panels were immersed in the electrolyte and electrolytically treated as the cathode under the current conditions noted in Table I, which varied from 100 to 400 amperes per square foot for 1-2 seconds, to thereby deposit a corrosion resistant film thereon. The temperature of the electrolyte was varied between 100 and F.

A series of runs was conducted under varying conditions and the data recorded in Table I below. The amount of chromium in the film deposited on the test panels was determined by prior art analytical methods for both metallic chromium and chromium present in the form of chromium oxide wherein the chromium had a valence of plus 3. The percent of metallic chromium in the film was calculated and recorded.

The treated panels were also tested for corrosion resistance in a controlled atmosphere, and for lacquer adhesion by the phenolic wedge-bend test. In the corrosion or rust test, the treated panels were placed for five days in a chamber which was maintained at 65% relative humidity at F. The percent of the surface area which was covered with rust at the end of five days was determined and recorded in Table I.

In making the phenolic wedge-bend test, clean dry treated panels were coated with phenolic lacquer, the lacquer cured, and then the panels were fabricated by impact using the well known test equipment developed by the Continental Can Company. Scotch tape was then applied to the bent surface area and quickly stripped away to thereby remove any organic coating which was not tightly adhered. The grading system for this test procedure is 0 through 16. A rating of 0 indicates perfect adhesion of the lacquer and 16 indicates complete failure, i.e., all of the organic coating is removed by the Scotch tape. The ratings thus obtained are also recorded in Table I.

The above test is indicative of the degree of side seam adhesion which is exhibited by the treated blackplate surface. In instances where the phenolic wedge-bend test gave a value of 1-2 or less, then the treated blackplate surface had statisfactory side seam adhesion properties for the manufacture of containers by application of an organic cement such as an epoxy cement between the side edges to be joined.

The appearance of the treated blackplate panel from each run was also observed and the color recorded The 6 What is claimed is: 1. A method of improving the corrosion resistance of a ferrous metal article comprising electrolytically treating the article cathodically at a current density of about color varied from gray to silver-gray, as noted iii Table I. 5 50-800 amperes per square foot in an acidic aqueous TABLE I mg./sq. it.

Eleetro- Percent lyte Current, Rust Phenolic Temperamps of (percent Wedge Color of Run ature sq. it. Time 01* Or Total Surface Bend Deposited No. F.) (cathodic) (sec.) (oxide) (metal) (Cr"+0r) Area) Test Film 1 100 100 2 4. l 3. 44 1 1 Silver-gray. 2.. 100 200 2 3. 2 7. 4 69. 5 1 1 Do. 3- 100 360 1 3. 2 7. 05 69 2 1 Gray-silver. 4- 125 100 2 3. 0 3. 4 53 1 1 Silver-gray. 5 125 200 2 2. 55 7. 1 73. 5 1 Gray-silver. 6- 125 400 1 4. 95 5. 95 54. 5 2 2 Gray. 7. 150 100 2 2. 9 1. s5 39 2 1 Silver-gray. 8. 150 200 2 2. 65 4. 62 1 1 Gray. 9- 150 400 1 2. 05 5. 8 74 1 2 D0.

Upon reference to the data in Table I, it may be seen that the treatment of the invention is highly effective in preventing rust formation, as only one or two percent of the surface area was rusted after five days at 65% relative humidity. Additionally, the phenolic wedge-bend test values were 2 or less, whichis indicative of extremely good adhesion of the lacquer and also good adhesion when using a side seam cement. The panels were not discolored, and had a gray, silver-gray, or gray-silver appearance and thus could receive clear lacquer or enamel in the manufacture of containers therefrom.

A control was also run which was a clean but untreated blackplate panel. This panel was rusted over completely. Thus, the treatment of the invention is capable of imparting excellent corrosion resistance.

Example H The procedure of Example I was followed with the exception of using an electrolyte which did not contain sulfate ion (sulfuric acid). All of the other ingredients of the electrolyte and the quantities were the same. The tests performed and the test procedures were also the same. The results obtained for a series of runs similar to Example I are recorded in Table II.

electrolyte containing 15-250 grams per liter of a hexavalent chromium compound, about 25-200 parts per million by weight of chloride ion and about 150-2000 parts per million by weight of sulfate ion, the pH value of the electrolyte being less than 2.5 and the said electrolytic treatment of the ferrous metal article being continued for a period of time sufiicient to deposit a corrosion resistant chromium containing film thereon.

2. The ferrous metal article of improved corrosion resistance prepared by the method of claim 1.

3. The method of claim 1 wherein the ferrous metal article is cathodically treated over a period of time sufiicient to provide about 100-800 coulombs per square foot of treated surface area.

4. The method of claim 1 wherein the temperature of the electrolyte is about 70-175 F.

5. The method of claim 1 wherein the temperature of the electrolyte is about 70-175 F. and the ferrous metal article is cathodically treated over a period of time sufficient to provide about 100-800 coulombs per square foot of treated surface area.

6. The method of claim 1 wherein the electrolyte contains about 20-70 grams per liter of the hexavalent chromium compound, about -150 parts per million by TABLE II Electrolyte Current, Mg./sq. It, Phenolic Temper- Amps/ Percent Rust Wedge Color of ature sq. it. Time (Jr- Surface Bend Deposited F.) (cathodic) (sec) (oxide) Area Test Film 100 100 2 6.85 1 5 Blue-green. 100 200 2 8.9 1 8 Yellow. 100 360 1 8.0 5 11 Blue-green. 125 100 2 4.6 3 11 Purple-blue. 125 200 2 8.3 5 12 Yellow-green. 125 400 1 7.8 10 12 Bluegreen. 150 100 2 5.1 5 5 Blue-purple. 150 200 2 8.6 2 12 Blue-green. 150 400 1 8.9 1 12 Blue.

Upon reference to Table II, it may be noted that the amount of rust was substantially higher, and the phenolic wedge-bend test was entirely unsatisfactory. Thus, phenolic lacquers and enamels would not adhere sufiiciently tight. This would result in a high corrosion rate, and the underfilm corrosion rate would be too high for uses such as beverage container stock. Additionaly, the side seam adhesion properties would not be sufficiently good for the manufacture of containers therefrom using an organic cement. The treated blackplate product produced by this example would not be satisfactory for use as a container stock, whereas that produced in accordance with Example I would be very satisfactory.

weight of chloride ion and about 200-800 parts per million by weight of sulfate ion, the article is cathodically treated at a current density of about 100-600 amperes per square foot over a period of time suflicient to provide 200-600 coulombs per square foot, the pH value of the electrolyte is less than 2, and the temperature of the electrolyte is about 70-175 F.

7. The method of claim 1 wherein the electrolyte contains about 40-50 grams per liter of the hexavalent chromium compound, about -110 parts per million by weight of chloride ion and about 400-500 parts per million by weight of sulfate ion, the article is cathodically treated at a current density of about 200-400 amperes per square foot over a period of time sufficient to provide about 300-400 coulombs per square foot, the pH value of the electrolyte is less than 1.0, and the temperature of the electrolyte is about IOU-120 F.

8. The method of claim 1 wherein the hexavalent chromium compound is chromic acid.

9. The method of claim 8 wherein the ferrous metal article is cathodically treated over a period of time suflicient to provide about 100-800 coulombs per square foot of treated surface area.

10. The method of claim 9 wherein the eletcrolyte consists essentially of an aqueous solution of chromic acid, hydrochloric acid and sulfuric acid.

11. The method of claim 8 wherein the electrolyte contains about 20-70 grams per liter of chromic acid, about 50-150 parts per million by weight of chloride ion and about 200-800 parts per million by Weight of sulfate ion, the article is cathodically treated at a current density of about 100-600 amperes per square foot over a period of time suflicient to provide 200-600 coulombs per square foot, the pH value of the electrolyte is less than 2, and the temperature of the electrolyte is about 70-175 F.

12. The method of claim 8 wherein the temperature of the electrolyte is about 70175 F.

13. The method of claim 8 wherein the electrolyte contains about -50 grams per liter of chromic acid, about -110 parts per million by weight of chloride ion and about 400-500 parts per million by weight of sulfate ion, the article is cathodically treated at a current density of about 200-400 amperes per square foot over a period of time sufficient to provide about 300-400 coulombs per square foot, the pH value of the electrolyte is less than 1.0, and the temperature of the electrolyte is about -120" F.

14. The method of claim 13 wherein the electrolyte consists essentially of an aqueous solution of chromic acid, hydrochloric acid and sulfuric acid.

15. The method of claim 14 wherein the chromium containing film which is deposited on the article contains metallic chromium and chromium in the form of an oxide thereof, and the total weight of chromium in the film is about 2-25 milligrams per square foot.

No references cited.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner U.S. Cl. X.R. 20451 age UNITED STATES PATENT ()FFICE CERTIFICATE OF CORRECTION Patent No. 3, 475, 295 Dated October 28, 1969 Inventor) John R. Smith and James R. Bray It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 69, change "Additionaly" to Additionally-.

Column 6, line 50, before "Surface" insert Column 6, line 62, change "12" to l3.

SEALED FEB 1 7 1970 1 m Atteat: W

ILLIAH E. 2 Edward M. Fletcher, Ir. g nef mgngik.

Attesti ug Officer 

